In the fabrication of gears, a distinction is typically made between the so called soft machining and the hard machining. The soft machining is carried out before the hardening of the work piece. The hard machining, however, is carried out after the hardening.
After the soft machining, due to the subsequent hardening, deformations due to hardening may result. In order to achieve the desired geometry, contact properties and surface qualities, such gear wheels are thus often machine finished in the hard state.
FIG. 1 shows a schematic representation of an exemplary process flow 100. A general example for the machining of a gear wheel is concerned. Starting from a blank (box 101), the following soft machining steps are carried out in the example shown. For example, a (central) bore hole may be generated by turning (box 102). Then, the blank may be turned over for further machining (box 103). After the turning over, an anew machining by turning may follow (box 104). These steps are optional and are also called preform fabrication here. Other steps or alternative steps may also be carried out within the framework of the preform fabrication. At the end of the preform fabrication, the work piece is called gear blank or work piece blank K1. The step 102 or the steps 102 to 104 may be carried out for example in a so-called pre-machining machine or in several different machines.
Then, the gear cutting follows typically. In the context of a gear cutting machining, a predefined number of tooth spaces is carved out by means of the gear roughing (box 105) at the bevel gear wheel blank K1 in the approximate position and with approximate allowances using a rough machining tool. Then, the optional step of the teeth cutting finishing (box 106) may follow, which is performed using a finishing tool and which provides to the tooth spaces respectively the adjoining tooth flanks the desired shape with the predetermined accuracy by the finishing machining. However, it is also possible to carve out the tooth flanks in only one step (instead of the two steps 105 and 106).
The steps 105 and 106 may, for example, be performed in one and the same machine or machining station. The workpiece blank K1 then does not have to be re-clamped or transferred.
The previous steps are called soft machining, as indicated in FIG. 1.
Subsequently, a heat treatment (box 107) of the pre-fabricated workpiece K2 follows typically. This heat treatment typically does not count anymore to the soft machining. It serves to harden the pre-fabricated workpiece K2. Then, the already mentioned machine finishing (box 108) follows. In the machine finishing, which is also called hard fine machining herein, a hard machining of the tooth system is performed. Then, the gear wheel K3 is completed.
In the hard machining, end mills or ball-shaped head-like millers are employed to some extent today, which are slowly guided over the surfaces of the pre-manufactured tooth flanks to be finished. Investigations show that the surfaces, which have been finished in that way, may in certain circumstances show explicit processing traces and may have a bad contact pattern. This is the case in particular, when insufficient time is available for the finishing and/or when the tool is guided too quickly over the tooth flanks to be finished. Hydrodynamical lubrication problems may also result, for example, when two gear wheels, which have been finished in that way, are employed as a gear pair.
It is thus an object of the present invention to provide a method for hard fine machining of a pre-manufactured tooth flank of a gear wheel, which provides for a better tooth contact pattern or better surface properties of the tooth flanks and which features a good productivity.